Method for treating extremely small particles of plastic

ABSTRACT

A method for treating extremely small particles of plastic comprises providing a quantity of plastic particles having an average mean particle size ranging from about 0.0005 inch to about 0.05 inch in diameter, heating the plastic particles to a temperature sufficient to cause at least a portion of the plastic particles to adhere to one another, and forming the adhered plastic particles into pellets, said pellets having substantially the same average surface-to-volume ratio as the bulk, un-adhered plastic particles.

FIELD OF THE INVENTION

The present invention relates generally to a method for treatingextremely small particles of plastic. More particularly, the inventionis directed to a process for treating particles of plastic, to prepare astock material that is easier to handle than is a fine powder, yet whichsubstantially retains the processing, operability, and utility exhibitedby extremely small particles of plastic.

BACKGROUND OF THE INVENTION

It has recently been discovered that extremely small particles ofplastic exhibit unexpectedly superior processing properties for theproduction of new plastic articles. U.S. Pat. Nos. 6,734,234 and5,899,392, for example, disclose that extremely small particles of RPETproduced by comminuting RPET flakes may easily be decontaminated andrecycled to make plastic containers.

However, extremely small particles of plastic are difficult to transportand otherwise handle. Generally, the handling of fine powders requiresspecialized equipment such as customized vacuum conveying systems, densephase conveyors, crystallizers, dryers, bins, tanks, and augers. Suchhandling equipment is expensive, and costly to operate and maintain. Forthese reasons, manufacturers prefer to use larger plastic flakes orpellets and conventional handling equipment in the manufacture ofplastic articles, but thereby lose the benefits that could otherwise berealized by the use of extremely small particles of plastic.

It would be desirable to treat extremely small particles of plastic, toprepare a stock material that is easy to handle in conventionalprocessing equipment, yet which substantially retains the processingadvantages, operability, and utility exhibited by extremely smallparticles of plastic.

SUMMARY OF THE INVENTION

Accordant with the present invention, a process for treating extremelysmall particles of plastic to improve handleability surprisingly hasbeen discovered. The process comprises the steps of providing a quantityof plastic particles having an average mean particle size ranging fromabout 0.0005 inch to about 0.05 inch in diameter, heating the plasticparticles to a temperature sufficient to cause a portion of the plasticparticles to adhere to one another, and forming the plastic particlesinto pellets, said pellets having a substantially highersurface-to-volume ratio compared to bulk recycled or virgin resinpellets.

The inventive process is particularly useful for preparing pellets ofplastic whose processing advantages, operability, and utility areessentially identical to bulk extremely small particles of the plastic,for the ultimate production of plastic articles such as containers.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features that are considered characteristic of the inventionare set forth with particularity in the appended claims. The inventionitself, however, will best be understood from the accompanyingdescription of specific embodiments when read in conjunction with theattendant Drawings, in which:

FIG. 1 is a fragmentary elevational view, partly in section, ofapparatus for treating extremely small particles of plastic, including adevice for heating the plastic particles, extruding the heated, adheredplastic particles, and cutting the extrusions to form pellets;

FIG. 2 is a fragmentary sectional view taken along the line 2-2 of FIG.1; and

FIG. 3 is an enlarged fragmentary view of the forming passageways of theheated bushing illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENT

A quantity of plastic particles having an average mean particle sizeranging from about 0.0005 to about 0.05 inch in diameter is provided,according to the present invention. These extremely small particles ofplastic may be formed by grinding or otherwise comminutingreadily-available plastic flakes, utilizing conventional equipment suchas grinders, ball mills, impact grinders, cryogenic grinders,pulverizers, attrition mills, and the like. A preferred particle sizefor the plastic particles is about 0.01 inch in diameter (approximately300 microns). These powder-like plastic particles are very difficult toconvey and otherwise handle.

Plastics useful for practicing the present invention include, but arenot necessarily limited to, polyolefins such as polyethylene in all ofits forms (e.g., low density polyethylene, linear low densitypolyethylene, medium density polyethylene, high density polyethylene,etc.), polypropylene, polybutylene, and polyisoprene, including theirisomers, polystyrene, polyacrylates, polymethacrylates, polyvinylchloride, polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol,polyesters, polycarbonates, polyethers, polyamides, and the like, aswell as derivatives, blends, and copolymers thereof.

The plastic particles are heated to a temperature sufficient to cause atleast a portion of the plastic particles to adhere to one another.Referring now to the Drawings, and particularly FIGS. 1 and 3, there isshown apparatus, generally indicated by reference numeral 10, forforming the plastic particles, according to the present invention. Theapparatus 10 comprises a bushing 12 having a plurality of diepassageways 14 through which the plastic particles move to formbaculiform extrusions 18. Although circular cross-sectional diepassageways 14 are illustrated in the Drawings, resulting in pelletshaving diameters represented by the arrows 30 in FIG. 3, it will bereadily apparent to those ordinarily skilled in the art that othercross-sectional configurations (not shown) may be used.

The plastic particles are heated to a temperature at or slightly abovethe glass transition temperature (Tg) of the plastic from which theparticles were derived. Generally, the glass transition temperature forpolyethylene particles, for example, is greater than about 125 degreesCelsius. The particles are heated by frictional engagement until atleast a portion of them adhere to one another by point contacttherebetween. Thus, the particles essentially retain their overallshapes, and the interstitial spaces between the particles are likewiseretained in the adhered mass.

The adhered plastic particles are formed into pellets 20, said pellets20 having substantially the same average surface-to-volume ratio as thebulk, un-adhered plastic particles 16. Although cylindrical pellets 20are illustrated in FIG. 1, the term “pellet” is contemplated to includeall forms of sintered plastic particles 16 including, for example,flakes, spheres, cubes, misshapen chunks, and the like (not shown). Aswill be readily apparent to one ordinarily skilled in the art, theconfiguration of the “pellets” will be determined by the process andequipment used to heat and form same.

FIGS. 1, 2, and 3 illustrate that the die passageways 14 are providedwith an initial funnel-shaped inlet 22, to assist in the formation ofthe extrusions 18. Alternatively, the initial portions of the diepassageways 14 may be provided with differently-shaped inlets, or noneat all (not shown). The adhered plastic particles are urged byconventional, suitable means (such as a ram, rollers, forced gas,gravity, and the like) through the die passageways 14 in the directionof the arrow 24 shown on FIG. 3 to produce the extrusions 18 of adheredplastic particles. The frictional engagement between the individualparticles and the walls 22 creates sufficient heat energy so that theadjacent surfaces of the particles in contact with the walls 22 reachthe glass transition temperature and tend to adhere to one another. Thisadherence causes at least the outer layers of particles to form apellet-like shell around a central mass of unadhered particles formingthe extrusions 18.

The extrusions 18 comprising at least partially sintered plasticparticles are caused to advance to an adjacent section of the apparatus10 referred to as the relief section 26, where the walls of the diepassageways 14 tend to diverge in the configurations of elongateinverted cones as they extend toward the exits of the die passageways14.

The extrusions 18 are then caused to exit the die passageways 14. Acut-off knife 28 is caused to traverse the exit zone of the diepassageways 14, to sever the extrusions 18, thereby forming pellets 20of sintered plastic particles. The length of the pellets 20 isdetermined by the synchronization of the cut-off knife 28 traversal withthe velocity of the movement of the extrusions 18. The pellets 20 dropby gravity onto a collection surface which may comprise, for example, amoving conveyor, a collection bin, a conduit leading to other equipment,or the like.

Conveniently, the sintered pellets may then be conveyed by conventional,non-specialized equipment. This form of plastic retains the processingadvantages, operability, and utility of extremely small particles ofplastic, but eliminates the difficulties and costs associated with thetransport and handling of powder-like materials. The pellets mirror thecharacteristics of the particles of plastic; in that the pellets exhibita very fast drying time, accelerated diffusion for the removal ofcontaminants or the infusion of adjuvants, faster build rates forintrinsic viscosity during subsequent solid-stating processes, etc.

The bushing 12 may be heated by conventional means such as, for example,electrical resistance heating, flame heating, infrared radiant heating,hot gas heating, and the like. The bushing 12 is heated in such a mannerso that the bulk plastic particles 16 contained within the bushing 12attain, by friction between the particles or by conduction and/orconvection of the thermal energy from the bushing 12, a temperaturesufficient to cause at least a portion of the plastic particles 16 toadhere to one another.

The invention is more easily comprehended by reference to specificembodiments recited hereinabove which are representative of theinvention. It must be understood, however, that the specific embodimentsare provided only for the purpose of illustration, and that theinvention may be practiced otherwise than as specifically illustratedwithout departing from its spirit and scope.

1. A process for treating extremely small particles of plastic,comprising: providing a quantity of plastic particles having an averagemean particle size ranging from about 0.0005 inch to about 0.05 inch indiameter; heating the plastic particles to a temperature sufficient tocause at least a portion of the plastic particles to adhere to oneanother; and forming the adhered plastic particles into pellets, saidpellets having substantially the same average surface-to-volume ratio asthe bulk, un-adhered plastic particles.
 2. The process according toclaim 1, wherein the plastic comprises polyolefin, polystyrene,polyacrylate, polymethacrylate, polyvinyl chloride, polyacrylonitrile,polyvinyl acetate, polyvinyl alcohol, polyester, polycarbonate,polyether, polyamide, or a derivative, blend, or copolymer thereof. 3.The process according to claim 1, wherein the average mean particle sizeof the plastic particles is about 0.01 inch in diameter.
 4. The processaccording to claim 1, wherein the heating is accomplished by electricalresistance heating, flame heating, infrared radiant heating, or hot gasheating.
 5. The process according to claim 1, wherein the heating isaccomplished by frictional engagement between the particles.
 6. Theprocess according to claim 1, wherein at least some of the plasticparticles are heated to a temperature greater than or equal to theirglass transition temperature.
 7. The process according to claim 1,wherein the plastic particles are heated to a temperature greater thanabout 125 degrees Celsius.
 8. The process according to claim 1, whereinthe pellets are formed into cylinders, flakes, spheres, cubes, ormisshapen chunks.
 9. The process according to claim 1, wherein thepellets are formed by passing the heated plastic particles through diepassageways.
 10. A process for treating extremely small particles ofplastic, comprising: providing a quantity of plastic particles having anaverage mean particle size about 0.01 inch in diameter; heating theplastic particles, by frictional engagement, electrical resistanceheating, flame heating, infrared radiant heating, or hot gas heating, toa temperature greater than or equal to their glass transitiontemperature, to cause at least a portion of the plastic particles toadhere to one another; and forming the adhered plastic particles intopellets in the form of cylinders, flakes, spheres, cubes, or misshapenchunks, by passing the heated plastic particles through die passageways,said pellets having substantially the same average surface-to-volumeratio as the bulk, un-adhered plastic particles.
 11. The processaccording to claim 10, wherein the plastic comprises polyolefin,polystyrene, polyacrylate, polymethacrylate, polyvinyl chloride,polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol, polyester,polycarbonate, polyether, polyamide, or a derivative, blend, orcopolymer thereof.
 12. The process according to claim 10, wherein theplastic particles are heated to a temperature greater than about 125degrees Celsius.
 13. A process for treating extremely small particles ofplastic, comprising: providing a quantity of plastic particles,comprising polyolefin, polystyrene, polyacrylate, polymethacrylate,polyvinyl chloride, polyacrylonitrile, polyvinyl acetate, polyvinylalcohol, polyester, polycarbonate, polyether, polyamide, or aderivative, blend, or copolymer thereof, having an average mean particlesize about 0.01 inch in diameter; heating the plastic particles, byfrictional engagement, electrical resistance heating, flame heating,infrared radiant heating, or hot gas heating, to a temperature greaterthan about 125 degrees Celsius, to cause at least a portion of theplastic particles to adhere to one another; and forming the plasticparticles into pellets in the form of cylinders, flakes, spheres, cubes,or misshapen chunks, by passing the plastic particles through diepassageways, said pellets having substantially the same averagesurface-to-volume ratio as the bulk, un-adhered plastic particles.